RFR: 8303186: Missing Classpath exception from Continuation.c

[Tyler Steele]

This PR adds the Classpath exception to the header file of `src/java.base/share/native/libjava/Continuation.c`.

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Commit messages:
 - Adds classpath exception

Changes: https://git.openjdk.org/jdk/pull/12747/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12747&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303186
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12747.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12747/head:pull/12747

PR: https://git.openjdk.org/jdk/pull/12747